LeetCode94 二叉树的中序遍历（栈）

题目链接：leetcode94

题目大意

给定一个二叉树，返回它的中序 遍历。

非递归实现

解题思路

栈+模拟

大致的思路就是不断往左子树切换，直到左边为空后把当前的节点输出，然后转到右子树。如果右子树为空说明需要回溯，弹栈即可。

代码实现

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> stk;

        TreeNode *curpoint = root;
        while (curpoint) {
            if (curpoint->left) {
                stk.push(curpoint);
                curpoint = curpoint->left;
            } else {
                res.push_back(curpoint->val);
                if (curpoint->right) {
                    curpoint = curpoint->right;
                } else { //右子树遍历完毕
                    curpoint = NULL;
                }
            }
            while (!curpoint && !stk.empty()) { //回溯切换到右子树中
                res.push_back(stk.top()->val);
                curpoint = stk.top()->right;
                stk.pop();
            }
        }
        return res;
    }
};